When subjected to normal catalytic hydrogenation processes, residual petroleum oils cause rapid catalyst deactivation. This is due for the most part to the presence of the large quantities of pentane insoluble asphaltenes in the residual petroleum oils.
The term "pentane insoluble asphaltenes" hereinafter called asphaltenes is herein used to mean the pentane insoluble high boiling asphaltenes that are sheet-like structured materials containing the bulk of the metal components in the oil. The asphaltenes are readily adsorbed on the surface of the catalysts and the metals, such as vanadium and nickel, which primarily associated with the asphaltenes, are deposited on the catalyst particles thus blocking the catalyst pores and preventing other molecules from coming into contact with the active catalyst sites. Asphaltenes are responsible for the rapid deactivation that is usually observed in residual oil hydrodesulfurization. Furthermore at high temperatures, the asphaltene molecules polymerize and cause plugging of the catalyst bed in high conversion operations.
In order to avoid this rapid catalyst deactivation due to the asphaltenes, it is a practice in the prior art to only treat a portion of the resid feed. The vacuum gas oil is separated by distillation, hydrotreated and then blended back with the residual oil material. This prevents the contacting of the catalyst with asphaltenes. A major problem arises under the foregoing method when the production of a low sulfur fuel is desired, since the sulfur contained in the asphaltene molecules represents a significant portion of the total sulfur and has not been removed.
It has therefore become necessary to discover a method which will economically hydrotreat asphaltene containing residual oil feedstocks without rapid deactivation of the catalysts employed.